1. Field of the Invention
The present invention relates to wireless IC devices, and, in particular, to a wireless IC device used in an RFID (Radio Frequency Identification) system.
2. Description of the Related Art
In recent years, RFID systems have been developed as an article management system in which communication is established in a non-contact manner between a reader/writer that generates an induction field and an IC tag (hereinafter referred to as a wireless IC device) which is affixed to an article and which stores predetermined information, whereby information is exchanged. An example of a wireless IC device used in an RFID system is described in “Musen IC Tagu Katsuyo-no Subete” (“All About Wireless IC Tags”), Nikkei BP Mukku-sha, pp. 112-126.
A wireless IC device of the above-described type includes a wireless IC chip mounted on a strap substrate, and the strap substrate is connected to an antenna substrate. The wireless IC chip and the strap substrate are directly electrically connected to each other by an Au bump, and the strap substrate and the antenna substrate are directly, electrically connected to each other.
However, since, in the wireless IC device, a center frequency and impedance are set on an antenna side, in order for the device to be matched to a plurality of frequencies and impedances, it is necessary to provide separate antennas for the individual frequencies and impedances. Accordingly, the wireless IC device is not suitable for limited production of diversified products.
Also, since the strap substrate and the antenna substrate are directly electrically connected to each other, a heating step is required. Thus, materials which can be used for the antenna substrate are limited to heat resistant materials. In addition, when a junction location between the strap substrate and the antenna substrate shifts, the center frequency and impedance are changed. In particular, in frequency bands greater than the UHF band, even a minute change causes changes to the center frequency and the impedance. Thus, extremely high precision is required when manufacturing the wireless IC device.